In the zinc industry one of the most challenging problems is the avoidance of the formation of zinc ferrite and zinc sulfate during roasting with the production of high grade calcine from iron-bearing zinc sulfide concentrate, typically assaying about 50 to 55% Zn, 2 to 17% Fe, 0.02 to 1% Cu, 0.1 to 0.3% Cd and less than 2% Pb, so that it is not essential (1) to carry out a zinc plant residue treatment in order to extract the significant quantities of the zinc combined as zinc ferrite normally left uneffected by calcine neutral leach operations presently practised, and (2) to bleed off the system in order to maintain the sulfate balance unchanged.
The zinc ferrite problem stems from the presence of iron in the concentrate feed. Zinc oxide and ferric oxide react with each other at elevated temperatures (about 600.degree. C.) to form very stable zinc ferrite spinels (ZnO.Fe.sub.2 O.sub.3). This occurs whether the iron is in solid solution in the zinc sulfide concentrate or is present in a separate iron phase. The loss of zinc due to insoluble zinc ferrites is represented in the industry as the ferritization ratio, defined as the weight of zinc rendered insoluble by a unit weight of iron and generally reported to be about 0.5 to 0.53. During roasting at temperatures above 900.degree. C., some of the iron remains as hematite but most of this combines with zinc oxide to form zinc ferrite. The lack of solubility of ferrites means that the solids residue remaining after a neutral sulfuric acid leach contains a significant concentration of zinc which, if no steps were taken to recover it, would represent a loss to the process and significantly affect the overall efficiency of the process.
A further complication is introduced by the presence of sulfate in the calcine. Zinc sulfate is soluble in leach liquor, but the ratio of zinc oxide to sulfate in the calcine is limited by requirements of the leaching and electrolytic circuits owing to its conversion to sulfuric acid upon electrolysis. Generally, should the sulfate content of the calcine exceed a limit of about 2%, purging of the leach liquor would be required to prevent build up of acid in the system, also entailing a loss in zinc yield.
From the foregoing it is evident that the quality of zinc calcine for use in hydrometallurgical processes is complicated by the presence of zinc ferrite and zinc sulfate and reflects itself materially in the operation of the hydrometallurgical process. Obviously, therefore, a process which would obtain complete avoidance of the formation of both zinc ferrite and zinc sulfate would represent a marked improvement and breakthrough in the art.
These prior art problems are now overcome with the development of an effective partial desulfurization roasting technique that avoids the formation of ferrites and sulfates, and a downstream leach processing technique which allows oxide-sulfide mineral treatment to be readily combined, thus making possible the production of high quality calcine for use in hydrometallurgical processes without entailing practically any loss of zinc yield except for an anticipated small bleed off stream for maintaining the heavy metal impurity concentration in balance in the process circuits.
The partial desulfurization of iron-bearing metal sulfide ores or concentrates is described in detail in our copending U.S. patent application Ser. No. 829,927 filed Feb. 18, 1986. As described therein, iron-bearing metal sulfide ores or concentrates, particularly zinc sulfide concentrates, are treated to produce a partially desulfurized concentrate characterized by extremely low, if any, zinc ferrite and zinc sulfate contents. This process comprises passing a concentrate in contact with an oxidizing gas and controlling the residence time to effect a specific degree of sulfur elimination at a temperature below the sintering temperature of the concentrate, thereby retaining a sufficient amount of residual sulfide-sulfur in the partially-desulfurized concentrate throughout the roast to effect a selective oxidation of iron sulfide content in the concentrate to an acid-leachable iron compound in which iron is maintained substantially wholly in the ferrous state, and the conversion of a portion of the zinc sulfide content in the concentrate to zinc oxide, thereby producing a quality partially desulfurized concentrate for economical hydrometallurgical treatment to recover the zinc metal values therefrom.
By the practice of the process of our copending application, it is possible (1) to avoid the formation of zinc ferrites, (2) to avoid the formation of zinc sulfates, (3) to produce a very active calcine, and (4) to concomitantly produce a very high strength SO.sub.2 -laden off-gas substantially free of oxygen and suitable for economical production of sulfuric acid. Furthermore, it is possible by the practice of the above process to employ a much smaller and more compact roaster than the commercially-available ones for the same given capacities, or alternatively, to employ higher throughputs in existing roasters, as much shorter residence times are required to effect such a partial roast.
The invention is directed to a method for hydrometallurgical treatment of partially-desulfurized zinc concentrates containing iron substantially wholly in its ferrous state for the recovery of the free zinc values therefrom which, when coupled with the partial-desulfurization method, constitutes a much simpler, more efficient and commercially more economical process than the currently-existing `dead` roast-leach-electrowin process.